The present invention relates to valves in general, and more particularly to flow control valves especially suited for use in controlling the operation of steam turbines.
There are already known various constructions of flow control valves of the above type, among them such in which a valve member is slidably guided in a valve member guide arranged in the interior of a valve housing for movement toward and away from a valve seat which is provided in the housing, and in which a diffusing channel is arranged downstream of the valve seat.
A flow control valve of this construction is disclosed, for instance, in the Appendix "Dampfturbinen grosser Leistung" (Steam Turbines with High Capacity) to Siemens-Zeitschrift (Siemens Magazine) 41 (1967), pp. 75 and 76. Such flow control valves should, on the one hand, achieve a desired throttling effect as a result of the adjustment of the position of the valve member within its adjustment stroke and, on the other hand, cause as low energy losses as possible in the fully open, that is, inoperative position of the valve member. In addition, the forces needed for the displacement of the valve member between its positions should be kept relatively low, in view of the practical limitations on the structural dimensions and power of the associated displacement drives operative for displacing the valve member and maintaining the same in the desired position.
In the single seat valves which are most frequently used in the design and construction of steam turbines, the required displacement forces are kept relatively low by choosing the diameter of the valve member to be relatively small. However, since the valve seat has to have correspondingly small dimensions, there are encountered relatively high flow speeds in the area of smallest flow-through cross section which is situated at or immediately downstream of the valve seat, even when the valve member is in its fully open or inoperative position. In order to keep the energy losses in the conduits arranged downstream of the valve at a low level, the flow speed is then decelerated with minimum losses in the diffusing channel disposed downstream of and immediately following the valve seat or the smallest flow-through cross section area. When the internal configuration of the flow control valve is as described above, so that it can be considered to constitute a converging-diverging channel, then the lowest pressure in the medium flowing through the flow control valve occurs at the area of smallest flow-through cross section, and the pressure then increases as the medium flows in the downstream direction through the diffusing channel.
When the flow of the medium, such as steam, through the flow control valve is to be throttled, for controlling or regulating the operation of a machine, suh as a steam turbine, arranged downstream of the flow control valve, the valve member is displaced out of its inactive position toward the valve seat. This results in a restriction of the flow-through cross-sectional area available for the flow of the medium between the valve member and the valve seat, but also in a change in the flow pattern of the medium downstream of the valve seat and in the diffusing channel. More particularly, the flow of the medium through the diffusing channel becomes detached from the surface bounding the diffusing channel. This phenomenon becomes progressively more pronounced as the valve member approaches the valve seat. The excessive speeds in the stream of the medium are then reduced by turbulence in the stream. As the degree of throttling of the stream of the medium flowing through the flow control valve increases, there are increasingly encountered supersonic speeds in the flow of the medium downstream of the valve seat. Such supersonic speeds are then reduced by surging shocks. However, both of the above-discussed forms of flow deceleration or production of losses, that is, either by turbulence or by surging shocks, cause a highly destablized flow pattern with a pronounced macroturbulence, with pulsation, and with to and from oscillation of the flow of the medium which has initially proceeded through the region of and immediately downstream of the valve seat in a streamlined fashion. The pressure variations resulting from these phenomena then act as undesirable oscillatory excitations on the valve member.
Particularly pronounced oscillations can occur even when the degree of throttling is relatively low. It is characteristic for the flow through the diffusing channel that the beginning or upstream end of the detachment of the flow from the surface bounding the diffusing channel extends along an irregular and constantly changing course, inasmuch as stochastic exchange occurs between the flow which is still attached and the flow which already became detached. The disruptions in the gradual pressure increase in the diffusing channel, which result from the above conditions, and the pronounced pressure shocks or pulses resulting therefrom, also act as oscillatory excitations on the valve member.